This invention relates to a polymer electrolyte membrane, and more specifically to a membrane for use in polymer electrolyte fuel cells and related applications.
Fuel cell technology provides for the combining of hydrogen protons with oxygen from air or as a pure gas. The process is accomplished utilizing a proton exchange membrane (PEM) sandwiched between two electrodes, namely an anode and a cathode. Membrane materials typically used for polymer electrolyte membrane fuel cells (PEMFCs) are Nafion(copyright) perfluorinated sulfonic acid polymers. Perfluorinated sulfonic acid polymers are believed to undergo microphase separation, which means that the hydrophilic sulfonic acid groups associate into separate regions from the perfluorocarbon polymer backbone. The backbone region is hydrophobic, and is not physically cross-linked, which means that chain mobility is not restricted severely. When the membrane is hydrated, water molecules enter the hydrophilic regions, increasing their size and shape, as well as membrane ionic conductivity. The effect of these dynamic results is a fairly narrow operating window for maximum fuel cell performance. At low levels of hydration, that is, low humidity and temperatures over 80xc2x0 C., the proton conductivity of Nafion(copyright) drops significantly. Other problems with Nafion(copyright) membranes are high cost, high osmotic drag of water, and high methanol permeability.
One of Nafion""s strengths as a fuel cell membrane is that it forms a microphase separated architecture upon film formation. Specifically, the film is composed of hydrophilic (water-loving) ionic xe2x80x9cclustersxe2x80x9d or xe2x80x9cchannelsxe2x80x9d dispersed in a hydrophobic matrix. At the same time, because of the lack of covalent cross-linking, the optimal structure for best performance can not be fixed, that is, the proton conductivity, channel size, and degree of hydration are dynamic and change with operating conditions. Another problem with Nafion(copyright) membranes is that protons need water or other similar functional groups in order to migrate through the membrane. There are no additional hydroxyl groups in Nafion(copyright) which can carry out this function. The ether oxygen atoms in the channels are flanked by strongly electron withdrawing CF2 groups, which render the lone pairs of electrons on the ether oxygen much less capable of sharing with a traveling proton.
In general, current approaches to new membrane materials are to add sulfonic acid groups to pre-formed aromatic polymers. There are some problems with this approach. First, the acidity of these sulfonic acid groups is usually much less than the fluorosulfonic acid groups in Nafion(copyright), making it more difficult to achieve comparable proton conductivities without resorting to extremely high degrees of sulfonation, which can lead to mechanical and solubility problems with the film. Second, there is no guarantee that sulfonation will result in a channel structure in a film. Highly aromatic, rigid polymers such as polyimides, and polybenzamidazoles, etc., for steric reasons may not be able to adopt the necessary configurations for good proton mobility through the film. Third, water is still necessary for proton transfer in these films, as there tends to be no other functional groups present which can hydrogen-bond with the proton and facilitate its transport.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art. New block copolymers proposed herein would provide for an improved membrane material for use in fuel cell devices, and in particular in polymer electrolyte membrane fuel cells. Accordingly, it is an object of the present invention to provide a new and improved polymer electrolyte membrane characterized by a three-dimensional structure with pores aligned and functionalized for the efficient transport of protons without the need for significant additional hydration.
It is another purpose of the present invention to provide for a new and improved polymer electrolyte membrane in which methanol permeability is reduced.
It is yet another purpose of the present invention to provide for a new and improved polymer electrolyte membrane with improved thermal and mechanical stability.
It is still another purpose of the present invention to provide for a new and improved polymer electrolyte membrane for use in fuel cell applications that require higher methanol concentrations, thereby providing for improved fuel utilization, enhanced cathode catalytic activity, and reduced system complexity for water recovery.
Briefly, to achieve the desired objects of the instant invention in accordance with a preferred embodiment thereof, provided is a polymer electrolyte membrane and a method of fabrication comprised of a hydrophobic hydrocarbon region, a hydrophilic region containing covalently bound acid functional groups and protic functional groups. The hydrophobic hydrocarbon region and the hydrophilic region are covalently bound to form a single polymer molecule.